The present invention relates to an operating medium for a condensation nucleus counter for internal exhaust gases from combustion engines and a condensation nucleus counter with an operating medium according to the invention.
Exhaust gases from an internal combustion engine contain solid particles in the nm range which are too small for it to be possible to detect them directly by optical means. In order to make such solid particles measurable, so-called condensation nucleus counters are frequently used in which the exhaust gas is passed through a supersaturated atmosphere. The supersaturated atmosphere is generated for example by saturating the exhaust gas with vapors from an operating medium and subsequently cooling it. The solid particles then serve as condensation nuclei onto which the supersaturated operating medium is condensed, which leads to a growth of the condensation nuclei. Such a condensation nucleus counter is known for example from U.S. Pat. No. 4,790,650 A or WO 12/142297 A1. The size of the solid particles from which this condensation process takes place is dependent upon the supersaturation and is designated as the Kelvin diameter. The smaller the Kelvin diameter is for a specific supersaturation, the smaller the solid particles can be, which lead to the condensation of operating medium. According to guidelines, for example statutory requirements, for exhaust gas the particle size range from greater than 20 nm, typically 23 nm, to 2.5 μm is to be detected and the exhaust gas is to be conditioned to a temperature of <35° C. before the saturator. Due to the condensation the size of the particles increases (to approximately 5 μm), and the particles can then be individually optically detected, for example with optical particle counters based on scattered light. For the detection and counting of individual particles, the particles must have a sufficient size in the μm range.
The currently standard operating medium for condensation nucleus counters for the measurement of solid particles in exhaust gases from internal combustion engines is 1-butanol (n-butanol), as described for example in EP 2 208 983 B1. The primary disadvantage of this operating medium is its chemical reactivity with the exhaust gas. With acidic exhaust gas components the alcohol forms esters which as a consequence are enriched in the wick elements of the condensation nucleus counter and lead to a reduction of the gas saturation. A further practical disadvantage is a flashpoint of ˜37° C., that is to say, in the range of the desired temperature.
In WO 01/31312 A1 an abundance of possible operating media for a condensation nucleus counter are examined, wherein the main focus in this case is aimed at the detection of very small molecules (smaller than 3 nm) for the chemical analysis of chemical substances. In order in this case to achieve the smallest possible detection limit, the system is geared to the smallest possible Kelvin diameters. Soot or other exhaust gas particles which are significantly larger than the molecules studied in WO 01/31312 A1 do not play any part here. In this case it is stated that glycol is the most suitable operating medium for such applications because it allows the smallest Kelvin diameters. In addition to a large number of further chemical substances which are conceivable in principle, alkanes, and specifically hexane, heptane, octane and nonane, are mentioned as operating medium, but these all enable poorer Kelvin diameters and therefore are not described in WO 01/31312 A1 as preferred operating medium for these applications.
Furthermore WO 01/31312 A1 describes a method for selecting an operating medium from a group of chemical substances (for example alkanes) as the most suitable operating medium. For this purpose the relative dielectric constant ∈r of the operating medium should be used and the operating medium with the highest dielectric constant in the chemical group are selected. The relative dielectric constant is a parameter which is known for the individual substances, for example from corresponding tables or specifications. Also by this criterion alkanes are categorized according to WO 01/31312 A1 as the least suitable operating media, because the relative dielectric constant thereof at ∈r˜2 is lower by more than one order of magnitude than that of the preferred operating medium glycol (∈r˜41) or glycerol (∈r˜47).
An alternative used in the field of atmospheric research is the use of water as operating medium (see also for example WO 01/31312 A1). However, for the present application in exhaust gases from internal combustion engines water cannot be used, since water does not grow sufficiently reliably on soot particles in the exhaust gas. In addition, because of the high diffusivity of water vapor in air, water would necessitate a fundamentally different system whereby water cannot be used in conventional condensation nucleus counters for exhaust gas. Thus water does not constitute a reasonable alternative as operating medium for the present application.
The use of perfluorinated compounds, in particular perfluoro-N-trialkylamines (for example perfluoro-N-tributylamine, Fluorinert FC-43), as operating medium for a condensation nucleus counter is known for example from U.S. Pat. No. 7,777,867 B2, Advantages of this operating medium are the excellent chemical inertness and the non-combustibility. However, a disadvantage of these operating media is the high density due to which conveying (and corresponding gas saturation) in condensation nucleus counters of specific design, for example with a vertical wick element, is not possible, which makes this compound usable only with limitations. Moreover, perfluorinated compounds are expensive and also potentially environmentally harmful, which makes the handling of such compounds difficult.
RU 2 237 882 C1 describes a method for determining the particle concentration of aromatic compounds in a gas by means of a nephelometer, wherein the aromatic compounds are initially converted by means of ozone to condensation nuclei which are then enlarged to particles by the use of tetradecane or heptadecane as operating medium by condensation. However, the tetradecane and heptadecane described therein are unsuitable as operating medium for a condensation nucleus counter for the particle measurement in exhaust gases from internal combustion engines, since in the desired operating temperature range they have a vapor pressure which is too low by an order of magnitude in order to produce an atmosphere which is sufficiently saturated or supersaturated with operating medium which is necessary for proper functioning. An increase in the operating temperature in order to achieve a sufficient saturation would be technically possible but, firstly, would not comply with the statutory and normative requirements of such devices for particle measurement in exhaust gases from internal combustion engines and, secondly, operating temperatures in the range of the flashpoints of the respective substances would be necessary, which would constitute a substantial safety hazard. Apart from that, in the operating temperature range of the condensation nucleus counter these alkanes are solid or do not have sufficiently low viscosity, which likewise rules out use thereof for the desired application.
Therefore an object of the present invention is to provide a suitable operating medium for a condensation nucleus counter for internal combustion engine exhaust gases.